An RF switch can be achieved by deflecting a metal membrane with an applied voltage so that the capacitance between two metal electrodes is dramatically changed. Fundamentally, such a switch is a reactive device so that the switch conducts RF signals when the capacitance is high and the capacitive reactance is low; i.e., ##EQU1## where X.sub.c is the capacitive reactance,
f is the RF frequency, and PA1 c is the capacitance of the switch. PA1 A is the area of either of the two metal electrodes PA1 s is the spacing between the two electrodes, and PA1 C is the capacitance. PA1 1. the need to fabricate tall posts to support the membrane PA1 2. a requirement for a relatively large voltage to pull down the membrane to activate the switch, and PA1 3. the stress placed on the membrane material when it is pulled down.
A thin dielectric can be used to separate the two electrodes so that a DC bias can be applied and maintained between them. The capacitance is a function of the area of the electrode and the spacing between the two metal electrodes; i.e., ##EQU2## where .epsilon. is the dielectric constant for the insulator
FIGS. 1 and 2 show a basic conventional MEMS switch mechanism for the OFF and ON conditions, respectively.
FIG. 1 shows a conventional MEMS RF switch in the OFF state. The switch structure is built on the chosen substrate 10 material and consists of two dielectric (insulator) posts 12. These posts have been constructed of both inorganic and organic polymer materials, both of which have problems. Problems with inorganic dielectric posts have been known to be related to stresses encountered with nitride or oxide layers in excess of a few microns thick. Organic polymers may be used as the post material but they tend to be less rigid and prone to degradation with time and environmental exposure. These dielectric posts support the flexible metal membrane 14 which is one plate of the capacitor. The second plate of the capacitor, the bottom electrode 16, is constructed on the surface of the substrate 10. A thin insulator, dielectric 18, is then placed on top of bottom electrode 16. An electrical connection is also made to the bottom electrode 16 for applying a DC bias 20, shown in the OFF state, to control the switch. Finally, connections are made for the RF input 22 signal and the RF output 26 signal. A fixed capacitor 24 is used to couple the switch structure to the RF output 26. In this state, there is no DC bias on the bottom electrode 16 and the membrane 14 is relaxed leaving a large separation between the two metal electrodes. This provides a low capacitance and high reactance condition which results in an OFF switch for RF signals.
FIG. 2 is the same structure as in FIG. 1, but now a DC bias 20 has been applied to the bottom electrode 16 to turn the switch ON. As shown, membrane 14 is now flexed down against the dielectric 18. This minimum separation between the two metal electrodes, membrane 14 and bottom electrode 16, yields a high capacitance and a low reactance resulting in an ON switch for RF signals.
Several of the problems associated with conventional MEMS RF switches include:
Representative prior structures are discussed in U.S. Pat. Nos. 5,578,976; 5,367,136; and 5,258,591. None of these patents disclose or suggest the novel features of the present invention.